In an arrangement known under the term readout electronics or front end electronics, the analog voltage input signal is applied to an analog to digital converter (ADC). Such readout electronics are used to generate a digital voltage signal from the input signal which can then be further processed digitally. Known devices with readout electronics can be, for example, multimeters, instrument transformers or sensor interfaces.
In order to be able to guarantee a highly reliable digital signal, readout electronics can be required to exhibit high initial accuracy, as well as high temperature and ageing stability. In addition, the high accuracy needs to be ensured over the whole specified temperature range and over a predefined time period, where the predefined time period is usually the time between a manual calibration of the readout electronics and the next calibration. During manual calibration, the manufacturer of the readout electronics applies an external standard reference voltage to the voltage signal input of the arrangement and uses the resulting digital signal to estimate and correct the gain error of the arrangement. The time period between two manual calibrations as well as the operating temperature range can be extendable by enabling the arrangement for so-called self-calibration. This is, for example, known under the term auto-calibration from the 3458A Multimeter by Agilent Technologies, as is described in the corresponding Calibration Manual, Manual Part Number 03458-90017. For the auto-calibration, the 3458A is equipped with internal reference standards, and the estimation and correction of gain errors is performed automatically whenever the auto-calibration function is invoked. During auto-calibration, the multimeter cannot be used for measurement purposes.
U.S. 2009/0315537 A1 discloses a sampling module for sampling an analog characteristic of a power transmission system. The sampling module includes a scaling circuit that reduces the magnitude of the analog characteristic, and an ADC which produces a 1-bit serial digital data stream from the reduced analog characteristic. The 1-bit serial digital data stream is digitally filtered by a digital filter and then resampled by a resampling module, where the sample rate of the resampling module is chosen in dependence on the further processing requirements of the particular analog characteristic. It is suggested that the scaling circuit could be calibrated, without giving any details for how the calibration could be implemented. Handling of signal errors which were introduced by the ADC can be not mentioned.
FIG. 1 shows a schematic diagram of known readout electronics with a self-calibration circuit to correct the gain error of the readout electronics. The gain of the readout electronics is summarized by the series connection of a gain block GA and converting means ADC having a gain GB. The input signal to the readout electronics is, under normal working conditions, an analog voltage input signal Vin, which is transformed into a corresponding digital output signal 2. For self-calibration purposes, internal reference means ref generate an analog reference voltage Vref, which is applied to an input switch 3 as well as to the converting means ADC. When self-calibration is desired, the input switch 3 needs to be operated in order to switch over from the analog voltage input signal Vin to the reference voltage Vref. The digital output signal 2 of the converting means ADC is then expected to be equal to the overall gain of the readout electronics GA·GB, since the digital output signal 2 equals the digitized value of the analog input signal 1 divided by the digitized value of the reference voltage Vref, i.e. it equals GA·GB·Vref/Vref. A discrepancy between the digital output signal 2 and the expected value GA·GB indicates the gain error of the readout electronics, the value of which is stored. Switch 3 is then operated back to apply the analog voltage input signal Vin, and the gain error is used to correct the digital output signal 2 generated from the analog voltage input signal Vin.
In the field of power transmission and distribution, voltages and currents can be measured by sensors for which it is desirable to perform self-calibration of the corresponding readout electronics without interrupting the current path of the analog voltage input signal Vin, so that the availability of the readout electronics is increased and manual interaction with the sensors is reduced. This type of self-calibration can also be called online self-calibration. In PCT/EP2011/001941, an arrangement is described for reading out an analog voltage input signal which at the same time delivers a signal which can be used to calibrate the arrangement without needing to disconnect the input signal. The main part of this arrangement can be superposition means which can be configured to generate a combined analog signal by superimposing the reference voltage onto the analog voltage input signal. Since the combined analog signal contains both the information of the input signal and the reference voltage, it is suitable to be used for self-calibration purposes while at the same time delivering the input signal to the ADC.